1. Field of the Invention
The invention relates generally to FM broadcase receivers and more particularly to noise muting circuits for such receivers.
2. Description of the Prior Art
In the prior art, various circuits have been used and proposed in efforts to mute or squelch noise in FM reception and prevent noise capture. The normal function of an FM receiver, through operation of its AFC and AVC circuits is to "lock" onto a received signal by adjusting its local oscillator frequency so that the received signal at IF will be centered within the IF pass band, and to hold a nominal overall gain. However, when no useful received signal is present or the receiver is detuned, "noise capture" can occur. This phenomenon is well understood by the skilled practitioners in this art.
One prior art approach to the noise capture problem has been the introduction of a substitute signal, for example at the IF center frequency. Such a signal may or may not be gated in accordance with the presence or absence of bona fide receivable signals.
The concept of introducing a substitute muting or squelch signal for this or similar purposes is described in the technical and patent literature. U.S. Pat. Nos. 2,395,737 (Hansell), 2,462,224 (Rheams) and 2,409,139 (Magnuski) all show substitute signals from auxillary or muting oscillators.
It has been found that the introduction of a substitute signal either on a gated or continuous basis at the IF pass band center frequency produces mixing by-products which appear in the audio band as "birdies" or "whistles" and is therefore undesirable. Introduction of a substitute signal within the IF pass band, other than at band center has the additional disadvantage of causing the AFC circuits to detune the receiver in attempting to counteract for such an off-center substitute signal. Still further, the discrimination would be unbalanced by such a signal with probable distortion of the bona fide signal at one extreme of the FM excursion.
It is also known that noise capture of an FM receiver can be caused by short path reflected signals, i.e. signals arriving at the receiver only slightly later than a direct path reception from the same source, since these multipath signal components are essentially coherent and frequently comparable in strength to the direct path desired signal. Such multipath signals fall in the category of noise which can "capture" the FM receiver and confuse its AFC circuitry. This confusion of AFC results from assymmetrical discrimination output.
The manner in which the present invention overcomes the prior art disadvantage by providing a novel muting arrangement will be evident as this description proceeds.